Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2:hang in atomic copy)

[Nigel Cunningham]

Hi.

On Wed, 2007-04-25 at 20:03 -0700, Linus Torvalds wrote:
> 
> On Thu, 26 Apr 2007, Nigel Cunningham wrote:
> > 
> > Sorry. I wasn't clear. I wasn't saying that suspend to ram has a
> > snapshot point. I was trying to say it has a point where you're seeking
> > to save information (PCI state / SCSI transaction number or whatever)
> > that you'll need to get the hardware into the same state at a later
> > stage. That (saving information) is the point of similarity.
> 
> Yes, they do both save information, but I'm not actually convinced they 
> would necessarily even save the *same* information.
> 
> Let's just take an example of USB, and to make things more interesting, 
> say that the disk you want to suspend to is itself over USB (not 
> necessarily something you _want_ to do, but I think we can all agree that 
> it's something that should potentially work, no?)

Agreed - it would be nice.

> Now, USB devices actually have per-connection state (at a minimum, the 
> "toggle" bit or whatever), and that's obviously something that will 
> inevitably *change* as a result of the device being used after 
> snapshotting (and even if not used, by the rediscovery by the first kernel 
> to boot), and we fundamentally cannot put the final toggle state in the 
> snapshot.
> 
> So in the snapshot-to-disk scenario, there are some pieces of data that 
> simply fundamentally *cannot* be snapshotted, because they are not 
> controller state, they are "connection" state.
> 
> So in that case, you basically know that you *have* to rebuild the 
> connection when you do the "snapshot_resume()" thing. So there's no point 
> in even keeping these kinds of connection states (the same is true of 
> keyboards, mice, anything else - it's how USB works).

Sort of agree - you might want to record some serial number that might
let you recognise it as the same thing at resume time when everything is
re-hotplugged (assuming it's even there then). Nevertheless, I don't
think that diminishes what you're saying.

> In contrast, in suspend-to-RAM, USB connections might just be things you 
> actually want to keep open and active, and you *can* do so, in ways you 
> simply cannot do with "snapshot to disk". In fact, if you are something 
> like an OLPC and actually go to s2ram very aggressively, you might well 
> want to keep the connection established, because it's conceivable that you 
> might otherwise lose keypresses etc issues)
> 
> See? There are real *technical* reasons to believe that the two "save 
> state" operations are really fundamentally different. There are reasons to 
> believe that a s2ram can actually happen while keeping some connections 
> open that cannot be kept open over a disk snapshot.
> 
> Do they *have* to be different? Of course not. For many devices the "save" 
> and "freeze" operations will likely all be no-ops, and there would be 
> absolutely no difference between suspending and snapshotting, because the 
> driver state already natively contains all the information needed to get 
> the device going again.
> 
> Equally, I don't doubt that in many drivers you'll have very similar "save 
> state" logic, but in fact I believe that in many cases that "save state" 
> logic will often just be a simple
> 
> 	pci_save_state(dev);
> 
> call, so it's literally the case that they will not be just shared between 
> the "suspend" and "snapshot" case, they'll be shared across all simple PCI 
> devices too!
> 
> But that doesn't mean that the functions to do so should be the same. You 
> might have
> 
> 	static int mypcidevice_suspend(struct pci_dev *dev)
> 	{
> 		pci_save_state(dev);
> 		pci_set_power_state(dev, PCI_D3);
> 		return 0;
> 	}
> 
> 	static int mupcidevice_snapshot(struct pci_dev *dev)
> 	{
> 		pci_save_state(dev);
> 		return 0;
> 	}
> 
> and who cares if they both have that same call to a shared "save state" 
> function? They're still totally different operations, and the fact that 
> *some* devices may save the same things doesn't make them any more 
> similar! See above why some devices might save totally *different* things 
> for a "snapshot" vs a "suspend" event.

No disagreement here.

> > I suppose that's another point of similarity - for snapshotting, the
> > same ordering is probably needed?
> 
> I agree that you're likely to walk the device list in the same order. The 
> whole "shut down leaf devices first", "start up root devices first" is 
> pretty fundamental.
> 
> But that's true of reboot and device discovery too. Should that ordering 
> mean that we should use the "discovery()" function and pass it a flag and 
> say "you shouldn't discover, you should snapshot or suspend now"? No. 
> Everybody agrees that device discovery is something different from device 
> suspend. The fact that it's done in a topological order and thus they bear 
> some kind of inverse relationship to each other doesn't make them "the 
> same".
> 
> > > And yes, the _individual_ "save-and-suspend" events obviously needs to be 
> > > "atomic", but it's purely about that particular individual device, so 
> > > there's never any cross-device issues about that.
> > 
> > No interdependencies? I'm not sure.
> 
> Well, we pretty much count on it, since we will *suspend* the devices at 
> the same time. So if they had interdependencies that aren't described by 
> the ordering we enforce, they are pretty much screwed anyway ;)
> 
> So yes, the device list needs to be topologically sorted (and you need to 
> walk it in the right direction), but apart from that we'd *better* not 
> have any interdependencies, or we simply cannot suspend at all.

Thanks for your reply.

Nigel

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